The invention relates to a method of driving a display device comprising a ferro-electric liquid crystal between two supporting plates and a system of picture elements arranged in rows and columns, the picture elements being constituted by picture electrodes arranged on the facing surfaces of the supporting plates and a system of row and column electrodes, at least one row of picture elements being selected via a row electrode during a line selection period, and data signals being presented via the column electrodes, the row of picture elements being brought to an extreme state by means of an auxiliary signal prior to presenting the data signals.
In this connection an extreme state is understood to mean such a state that the picture element is substantially completely transmissive or non-transmissive. This state is determined by the type of ferro-electric device; this will be explained hereinafter. The choice of the amplitude of the auxiliary signal also determines the speed at which the liquid crystal switches to this extreme state.
Such a method is used, for example, in display devices for display screen or television applications. Notably for television applications and non-volatile memory display devices, the use of ferro-electric liquid crystal materials appears to be attractive because they can realize much faster switching times than nematic liquid crystal materials.
Other advantages are less dependence on the viewing angle (because generally a smaller thickness is used and because of the orientation of the molecules in planes parallel to the walls), and greater contrast.
A method of the type mentioned in the opening paragraph is described in EP 0,197,242, in which the presentation of data signals in synchronism with a selection signal is preceded by so-called "blanking" pulses which must bring the liquid crystal to an initial state. To inhibit degradation of the ferro-electric material, the sign of the pulses used is periodically reversed.
The method described in said Patent Application is used in a display device comprising a so-called passive matrix; the said signals (selection signals, data signals, "blanking" pulse) are directly presented to the row and column electrodes. The state of a picture element is determined by the line and data voltages. Since data voltages are presented to the column electrodes also during non-selection of certain picture elements, voltages, possibly in combination with cross-talk signals, may be produced so that the desired transmission state (transmissive or non-transmissive in the embodiment of EP 0,197,242) is not reached or is lost.
The said Patent Application proposes to solve this problem by providing the ferro-electrical effect with a given threshold. Such thresholds cannot be realized or can hardly be realized in practice without loss of contrast occurring in applications using large numbers of lines. In the said Patent Application mention is only made of switching between the states "0" and "1" and the possibility of introducing grey scales in the devices shown is not referred to.
To obviate these problems U.S. Pat. No. 4,840,462, issued June 20, 1989 in the name of the Applicant proposes a method in which the picture elements are connected to the column electrodes via active switching elements during at least a part of the line selection period in order to present data signals to the picture elements, and in which the auxiliary signal is presented to the picture elements via the same switching elements at an instant before the start of the line selection at least during a period which is equal to the switching period of the ferro-electric medium and has such a duration and amplitude that the extreme transmission state is reached. After the line selection period the picture elements assume a transmission state which is substantially only determined by the data signal presented.
In this connection, the term "substantially only determined by the data signal presented" is understood to mean that the voltage present across the picture element during selection and the capacitance associated therewith is maintained and may possibly change to some extent due to leakage currents, but that no measures have been taken to generate a given voltage across the picture element during the non-selection period, as is shown, for example, in EP 0,176,763.
In the method as shown in the U.S. Pat. No. 4,840,462, grey scales are obtained in a reproducible manner in that the ferro-electric liquid crystal is brought each time to an extreme state for the purpose of selection. Consequently a successive charge compensation cannot take place in a so-called "active" matrix so that the growth of regions where transmission may or may not occur is inhibited. The auxiliary signal for obtaining an extreme state ("blanking") is presented before a selection signal, while a data signal coinciding with at least a part of the selection signal determines the ultimate transmission state (grey scale).
The said Application describes an embodiment of an auxiliary circuit in which the so-called "blanking" signals are presented six line selection periods before the line selection period coinciding with the data voltages by means of, inter alia, multiplex circuits and shift registers.
Dependent on the speed of the ferro-electric medium, the number of intermediate line selection periods may have a different value.